Method of bonding aluminum members



United States Patent 3,180,022 METHUD 0F BONDENG ALUMINUM NEE/ BEES BillN. Briggs, Canoga Park, and Sigmund 1F. Dajas, Reseda, Calif, assignorsto North American Aviation, Inc. No Drawing. Filed Sept. 2, 1960, Ser.No. 53,629 12 Claims. (Cl. 29-487) Our invention relates to a method ofjoining aluminum members, and more particularly to a diffusion bondingmethod of joining aluminum members.

Aluminum is widely employed as a cladding material for nuclear reactorfuel elements because of its low thermal neutron absorption crosssection and its generally satisfactory mechanical properties. However,the mechanical strength of aluminum is deficient for application in hightemperature reactors. Improved strength characteristics are particularlyrequired when the fuel is in a form, such as U0 which will not bear theload of the fuel assembly. Aluminum alloys having better hightemperature strength characteristics than aluminum and satisfactoryneutron cross sections are, therefore, being adapted for use in nuclearreactors. Prominent among the aluminum alloys being used for hightemperature reactor application, and for many other applications where agreater strength than that of aluminum metal is required, is the classof commercially-available aluminum alloys consisting of about 4-12 wt.percent aluminum oxide (A1 0 dispersed in an aluminum matrix. Unlessotherwise stated, the word aluminum, as used in this specification andin the appended claims, is defined to generically embrace both aluminummetal and aluminum alloys.

The bonding of aluminum members in strong, leaktight fashion is a severerequirement in fuel element fabrication. It is essential that closuremembers he securely bonded to prevent escape of fission products to theinterior of the reactor or the environment, and to protect the fuelagainst corrosive or erosive attack by Coolants. For example, a fuelelement may consist of a plurality of tubular aluminum alloy memberscontaining U0 and this assembly must be closed with a leak-tight andstrong end cap. Heretofore, the bonding of aluminum metal to aluminumalloy or aluminum alloy to aluminum alloy members has been accomplishedby high pressure, solid state bonding, for instance, at 30-80,000 p.s.i.This procedure is time consuming, may result in warpage of aluminummembers, and is not adapted to a production process Where thousands offuel rod end closures may be required for a single core loading. Weldingtechniques tend to destroy the elevated temperature properties of thealuminum-aluminum oxide alloys.

Accordingly, the principal object of our present invention is to providea method of joining aluminum members.

Another object is to provide such a method in which the resulting jointapproaches the strength of the parent material.

Another object is to provide a eutectic diffusion bonding method forjoining aluminum alloys.

Another object is to provide a method of forming reactor fuel rod endclosures which will remain gas leaktight at elevated temperatures.

Still another object is to provide such a method which can be rapidlyperformed using moderate pressures.

A further object is to provide such a method in which the boundary layerbetween the joined members is extremely thin and free of brittleintermetallic compounds.

A still further object is to provide such a method which is readilysuited as a production process.

Further objects and advantages of our invention will become apparentfrom the following detailed description and the appended claims.

3,18%,fi22 Patented Apr. 27, 1965 "ice In accordance with our presentinvention we have provided a method of joining aluminum members, whichcomprises providing on at least one mating surface a thin layer of abonding metal selected from the group consisting of silver, copper,gold, tin, and zinc, bringing the mating surfaces into intimate contact,and heating the resulting assembly at a temperature between the eutecticformation temperature and the melting point of the aluminum members,until said members are joined.

By the use of our method, aluminum members may be readily joined in abond which approaches the strength of the parent material and which doesnot diminish in strength at elevated temperatures. End closures on fuelrods have been effected which are gas tight and remain unaffected bytemperature cylcling at elevated temperatures over a prolonged period oftime. The bond formation appears to take place by the formation of aeutectic between the aluminum surfaces and the bonding metal, whichafter euectic formation diffuses away from the interface to leave anextremely fine juncture line.

In the practice of our invention a very thin layer of at least onebonding metal selected from the group consisting of silver, copper,gold, tin, and zinc, is applied to at least one of the surfaces to bejoined, but preferably only to one surface. We find that it is desirableto apply only a very thin layer of the bonding metal. For instance, thefilm is desirably not thicker than about 10* inches, while a film ofabout 10- to 10* inches is preferred. While thicker films may beoperably employed, such films may result in the formation ofintermetallic compounds at the interface which are brittle and hence ofdecreased strength. The film may be applied on to the surfaces bydeposition methods known to the art for the particular metals. Forinstance, films may be deposited by such convenient, controllable meansas vacuum and electrolytic deposition.

After at least one of the surfaces to be joined has been first cleanedby such conventional means as washings with organic reagents andalkaline and acid rinses, following which the bonding film is applied,the surfaces are brought together and maintained in intimate contact.This may be done by use of such conventional means as a collet or otherholding means. Only sufficient pressure need be applied to the holdingmeans to maintain the intimate contact during the heating period.

The temperature at which bond formation takes place is dependent uponthe particular eutectic bonding agent employed. The temperature willthus range between about 468 F. for eutectic formation between tin andthe aluminum member, 720 F. for zinc, 980 F. for gold, through about1012 F. for copper to about 1030 F. for silver. The maximum temperaturewill be a function of the melting or distortion points of the aluminum,this being about 1100 F. for aluminum metal and about 1200 F. for theabove aluminum alloy. It is desirable to exceed the eutectic formationtemperature for the particular eutectic material by about 30-60" F., inorder to promote diffusion of the eutectic into the parent material andformation of a thin interface which adds considerably to the quality ofthe resulting bond. For instance, a highly satisfactory temperature forsilver eutectic bonding is about 1060-1090 F., While a temperature ofabout 1080 F. is preferred. The heating step may be satisfactorilyconducted in either air or in an inert gas atmosphere. Maintainingintimate contact between the surfaces being joined is sufficient toprevent oxidation at the interface as the temperature is being raised,and air heating, by such means as an induction coil, is verysatisfactory as a production method.

We find that eutectic formation and diffusion into the parent materialto form a strong bond takes place at the above-indicated temperaturesrelatively rapidly, and this permits aluminum members to be joined in ashort period of time in a convenient manner. Thus, the members areheated at the above temperatures for a period of about 1 /2 to 10minutes, while a period of 2 /2 to 5 minutes is preferred. After theheating step is concluded, the now joined members are permitted to coolto the ambient temperature, preferably while still held by the holdingmeans. This is typically for a period of about 30 minutes. We find thatcooling while still maintaining the members in the holding means, whilenot critical, further promotes diffusion of the eutectic into the parentmaterial.

The following examples are offered to illustrate our invention ingreater detail.

Example 1 This example relates to forming a fuel rod end closure on atube having an ID. of 300:.003 and a wall thickness of .030 inch. Thetube was fabricated of an aluminum-443 wt. percent aluminum oxide alloyhaving the Alcoa commercial designation APM (M-257). The end plug wasmachined of the same material from a inch diameter, swaged rod. Thediameter of the portion of the end plug to be inserted in the tube was02971-002 inch.

The specimens were cleaned by degreasing with trichloroethylene liquidand vapor followed by cleaning with an aqueous caustic solution, rinsingwith water, treatment with nitric acid (50% by volume), and final waterrinsing and drying. The end plug was electroplated with silver from aplating bath comprising silver cyanide, 6 oz. per gallon, and potassiumcyanide, 12 oz. per gallon. The sample was electroplated for 30 secondsat 40 amps. per sq. ft., resulting in the deposition of about 10* to 10-inches of silver.

The end plug was put in the tube and placed under pressure in asix-segment collet having an open ID. of .375 inch. The assembly wasthen heated by induction heating to a temperature of 1060 F. -l100 F.Pressure was maintained bytightening the collet die at intervals ofapproximately 300 F. at 930 F., and after reaching temperature, in orderto maintain intimate contact upon expansion due to heating. Temperaturewas maintained for about 2 /2 minutes, after which the specimen waspermitted to cool in the die for about 20 minutes before removal. Thetube was similarly bonded on the other end. Both ends were found to behelium leak-tight by drilling one plug through for testing. Afterchecking, this hole was closed by hot knifing (1200 F.) using a copperplated aluminum weld rod inserted in the hole to facilitate closure.

The tube was then subjected to external pressure in an autoclave at 800F. When the pressure reached 2300 p.s.i., the tube failed but the endclosure was still intact and proved helium leak-tight.

The tubes were also subjected to pressure cycling tests at 750 F. Onespecimen was given 50 cycles from -1000 p.s.i.g. and remained heliumleak-tight. Another tube failed through the cladding but the end closureremained intact. Specimens also remained leak-tight after 90 days at1000 F. in an air furnace and after 1,000 hours in a terphenyl solutionat 750 F.

Example 2 The same as Example 1 except that copper was used as theeutectic bonding agent. The copper was electroplated on to the aluminumend plug from an aqueous plating bath containing copper cyanide, 6-8 oz.per gallon, free KCN, 1.21.5 oz. per gallon, and KOH, 3-4 oz. pergallon. About 10- inches of copper was deposited on the sample in 30seconds at 50 amps. per sq. ft. current density.

Testing of the resulting assembly using the methods of Example 1 showedthe resulting end closure to be helium leak-tight, and no leaks wererevealed after the pressure cycling, temperature cycling, and organicsoaking tests.

4; Example 3 The same as Example 1 except as indicated. In this examplethe aluminum-aluminum oxide tube was closed with an aluminum metal endplug. The closure was made utilizing a 0.250 inch O.D. tube having a0.020 inch wall. The end plug had a shoulder 0.250 inch in diameter x0.050 inch thickness for ease of placement. The area plated was 0.210inch in diameter x 0.500 inch long.

The aluminum metal end plug was plated by vacuum deposition with silverto a thickness ranging from 9X10 to 1.8 10- inches. The members werecleaned prior to plating as in Example 1. The components were thenassembled and placed in a collet die which was tightened to applypressure to the assembly. The die assembly containing the end closureassembly was then centered in an induction coil and heated for 2 /2minutes at a temperature of about 10601100 F. The die assembly wastightened every 30 seconds to maintain pressure while heating. Theclosure was allowed to remain in the die assembly until cool, about 25minutes. After removing the closure from the die, it was checked forhelium leaktightness at room temperature and found to be heliumleak-tight.

The closure was subjected to thermal cycles which ranged from 78 to 900F. and required 30 minutes per cycle. The specimen still remained heliumleak-tight.

The closure was then subjected to gas pressure after soaking at 750 F.for 30 minutes. The tube failed at 2300 p.s.i. with the closure stillintact. This specimen was then submitted to metallography which showedevidence of total interdiffusion at the bond interface and the bond tobe of unquestionable integrity.

The foregoing examples are merely illustrative rather than restrictiveof our invention. Variations may be made in assembly techniques by thoseskilled in the art within the scope of our invention. Therefore, ourinvention should be understood to be limited only as is indicated in theappended claims.

We claim:

1. A method of joining aluminum members which comprises providing a thinlayer of less than 10- inches of silver on at least one of the matingsurfaces of the aluminum members, contacting said mating surfaces and,while maintaining said members in intimate contact, heating theresulting assembly at a temperature between about 1030 F. and below themelting point of said aluminum members to form a eutectic of silver andaluminum at the interface of said members, and maintaining saidtemperature for a sufficient time to form a direct bond between thejoined members by diifusion of the formed eutectic away from theinterface into said members.

2. The method of claim 1 wherein said heating is conducted at atemperature of about 10601090 F.

3. A method of joining aluminum members which comprises providing alayer of silver of about 10- to 10* inches on one of the mating surfacesof said members, contacting said mating surfaces in intimate contact,and heating the resulting assembly at a temperature of about 1060 F. to1090 F. for a period of about 1 /2 to 10 minutes to form a eutectic ofsilver and aluminum at the interface of said members and to diffuse theformed eutectic away from the interface into said members, thereby todirectly bond said members together.

4. The method of claim 3 wherein said members are heated for a period ofabout 2 /2 to 5 minutes in air.

5. The method of claim 3 wherein said aluminum members are of analuminum alloy consisting essentially of about 4-12 weight percent A1 0and the remainder aluminum.

6. The method of claim 3 wherein the resulting assem bly subsequent toheating is permitted to air cool to the ambient atmospheric temperaturewhile said intimate contact is being maintained.

7. A method of joining aluminum members which comprises providing alayer of less than 10* inches thickness of a metal selected from thegroup consisting of silver, copper, gold, tin, and zinc on at least onemating surface, bringing the mating surfaces of said members togetherand, while maintaining said members in intimate contact, heating theresulting assembly at a temperature between the eutectic formationtemperature of said bonding metal and aluminum and the melting point ofsaid members to form a eutectic of said bonding metal and aluminum atthe interface of said members, and maintaining said temperature for asufficient time to form a direct bond between the joined surfaces bydifiusion of the formed eutectic away from the interface into saidmembers.

8. The method of claim 7 wherein said temperature range is between about500-1200 F.

9. The method of claim 7 wherein said assembly is heated for a period ofabout 1 /2 to 10 minutes.

10. The method of claim 7 wherein said assembly is heated in air.

11. The method of claim 7 wherein the film of said bonding material isprovided on the mating surface of said aluminum member by electrolyticdeposition.

12. The method of claim 7 wherein at least one of said aluminum membersis an aluminum alloy consisting essentially of about 4-12 Weight percentA1 0 and the remainder aluminum metal.

References Cited by the Examiner UNITED STATES PATENTS 2,633,633 4/53Bogart et a1 29498 X 2,790,656 4/57 Cook 29197 X JOHN F. CAMPBELL,Primary Examiner.

WHITMORE A. WILTZ, Examiner.

UNITED STATES PATENT OFFICE QERTIFICATE ()F CORRECTION Patent No.3,180,022 April 27, 1965 Bill N. Briggs et al.

It is hereby certified that'error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below...

Column 2, line 15, for "cylcling" read cycling column 3, line 17, for"500:.003" read .300i.003 line 40, for "300 F." read 300 F., column 6,under UNITED STATES PATENTS" after line 15 add the following:

2,798,845 7/57 Slomin et al. 29-504 UXR 2 ,9ll ,710 11/59 Kanter et a129-498 XR FOREIGN PATENTS 540,961 ll/4l Great Britain Q9-50l Signed andsealed this 3rd day of August 1965.

(SEAL) Attest:

ERNEST W. SWIDER I EDWARD J. BRENNER Attesting Officer Commissioner ofPatents

7. A METHOD OF JOINING ALUMINUM MEMBERS WHICH COMPRISES PROVIDING ALAYER OF LESS THAN 10**-3 INCHES THICKNESS OF A METAL SELECTED FROM THEGROUP CONSITING OF SILVER, COPPER, GOLD, TIN, AND ZINC ON AT LAST ONEMATING SURFACE, BRINGING THE MATING SURFACES OF SAID MEMBERS TOGETHERAND, WHILE MAINTAINING SAID MEMBERS IN INTIMATE CONTACT, HEATING THERESULTING ASSEMBLY AT A TEMPERATURE BETWEEN THE EUTECTIC FORMATIONTEMPERATURE OF SAID BONDING METAL AND ALUMINUM AND THE MELTING POINT OFSAID MEMBERS TO FORM A EUTECTIC OF SAID BONDING METAL AND ALUMINUM ATTHE INTERFACE OF SAID MEMBERS, AND MAINTAINING SAID TEMPERATURE FOR ASUFFICIENT TIME TO FORM A DIRECT BOND BETWEEN THE JOINED SURFACES BYDIFFUSION OF THE FORMED EUTECTIC AWAY FROM THE INTERFACE INTO SAIDMEMBERS.